1. Field of the Description
The invention relates to preform cutting elements for rotary drag-type drill bits, of the kind comprising a facing table of superhard material having a front face, a peripheral surface, and a rear surface bonded to the front surface of a substrate which is less hard than the facing table.
2. Description of Related Art
Such preform cutting elements usually have a facing table of polycrystalline diamond, although other superhard materials are available, such as cubic boron nitride. The substrate of less hard material is often formed from cemented tungsten carbide, and the facing table and substrate are bonded together during formation of the element in a high pressure, high temperature forming press. The forming process is well known and will not be described in detail.
Each preform cutting element may be mounted on a carrier in the form of a generally cylindrical stud or post received in a pocket in a body of the drill bit. The carrier is often formed from cemented tungsten carbide, the surface of the substrate being brazed to a surface on the carrier, for example by a process known as "LS bonding". Alternatively, the substrate itself may be of sufficient thickness to provide, in effect, a cylindrical stud which is sufficiently long to be directly received in a pocket in the bit body, without first being brazed to a carrier. The bit body itself may be machined from metal, usually steel, or may be molded using a powder metallurgy process.
In preform cutting elements of the above type the interface between the superhard table and the substrate may be flat and planar. However, the bond between the superhard facing table and the substrate may be improved by providing a configured non-planar interface between the rear face of the facing table and the front surface of the substrate, so as to provide a degree of mechanical interlocking between the facing table and substrate. It is also known to provide the rear surface of the facing table with an integral rearwardly extending peripheral rim which extends into a correspondingly shaped peripheral rebate in the substrate.
Such preform cutting elements are subjected to high temperatures and heavy loads when the drill bit on which they are mounted is in use down a borehole. It is found that as a result of such conditions delamination of the superhard facing table can occur, that is to say the separation and loss of the diamond or other superhard material over part or all of the front surface of the cutting element. The provision of a configured non-planar interface between the facing table and substrate, and the provision of a peripheral rim on the facing table, may reduce the tendency for delamination of the facing table to occur, but it is found that this can still sometimes occur with existing cutter interface configurations.
Studies have suggested that the impact loads which may result in delamination may be caused, at least in part, by torsional vibration of the drill string or by the phenomenon known as "bit whirl" where, if the borehole becomes slightly larger than the diameter of the drill bit, the bit may precess around the walls of the borehole in the opposite direction to the direction of drilling rotation of the bit.
Torsional vibration and bit whirl can both have the effect that cutters on the drill bit may momentarily be rotating backwards, i.e. in the reverse rotational direction to the normal forward direction of rotation of the drill bit during drilling. The effect of this reverse rotation on a PDC cutter may be to impose unusual loads on the cutter in directions which may increase the risk of delamination. Prior art designs of configured interface between the facing table and substrate of the cutting element may provide added strength against impact loads having components parallel to the front surface of the facing table and rearwardly parallel to the central axis of the cutting element. Normal impact loads imposed on the cutting element during forward rotation will generally have components in these two directions. However, existing designs provide little protection against impact loads having components in a forward direction with parallel to the central axis of the cutting element, that is to say in the direction of loads resulting from reverse rotation of the cutting element.
The present invention therefore sets out to provide an improved design of cutting element which may be, less susceptible to damage as a result of temporary backwards rotation of the cutting element.